The present invention relates to reciprocating plunger and piston-type pumps which are used, for example, in the well service industry. In particular, the invention relates to the connecting rod and crosshead assembly which converts the rotary motion generated at the power end of the pump into linear reciprocal motion which drives the plungers in the fluid end of the pump.
Prior art reciprocating pumps for the well service industry include a crankshaft which generates rotary motion and a slider crank mechanism to convert this rotary motion into linear reciprocating motion. The crankshaft is connected to a slider, which is commonly called a crosshead, by means of a connecting rod. The connecting rod is coupled to the crosshead with some type of joint which allows force to be transferred between these components in an oscillating fashion.
The most common joint design is that of a pin and bushing. The pin is typically referred to as a wrist pin. It is fixed in the crosshead and passes through a bushing or bearing in the connecting rod. In operation, force is transferred across the surface of the bushing to the mating surface of the wrist pin. This design works well when the unit load rating of the busing is sufficient to carry the total force of the connecting rod without distorting or rapidly wearing. However, the fact that the motion of the wrist pin relative to the bushing is oscillatory limits the ability of the bushing to develop a sufficient oil film for good lubrication.
Other prior art designs have been devised to provide a larger load bearing surface between the connecting rod and the crosshead. In one prior art joint, the end of the connecting rod forms a cylinder which engages a semi-cylindrical bushing that is embedded in the crosshead. However, this bushing is able to transfer the forces between the connecting rod and the crosshead only during the discharge stroke of the pump. Consequently, another mechanism must be used to transfer the forces between the connecting rod and the crosshead during the suction stroke of the pump.
One such mechanism comprises a small pin similar to a wrist pin which extends through both the connecting rod and the crosshead. However, this requires the crosshead to be wider than the connecting rod bearing surface, which necessarily increases the crosshead diameter and, consequently, the overall width of the pump. Furthermore, as the half bushing between the connecting rod and the crosshead wears in service, the small pin may begin to carry a portion of the force from the connecting rod that it is not designed to carry, and this may cause the pin to fail.
Thus, none of these designs for coupling the connecting rod to the crosshead provides an optimum means for transferring the forces between these components during both the discharge stroke and the suction stroke.